The best open source alternative to Box is Nextcloud, which is both free and Open Source. If that doesn't suit you, our users have ranked more than 100 alternatives to Box and many of them is open source so hopefully you can find a suitable replacement. Other interesting open source alternatives to Box are ownCloud (Freemium, Open Source), Seafile (Freemium, Open Source), Pydio (Freemium, Open Source) and SparkleShare (Free, Open Source).
Qabel is a free, published-source cryptography platform. The platform encrypts everything that is connected with it end-to-end, including Messenger, e-mail, industrial applications and individual solutions.
Internxt Drive offers a fully secure cloud storage experience. Files uploaded to Internxt Drive are client-side encrypted, and only the user holds the decryption key of these. Additionally, prior to being uploaded, files are fragmented. Servers only host encrypted shards.